Heretofore, a high temperature operating element has been manufactured using a so-called thick film circuit forming technique such as screen printing, as disclosed in Japanese Published Patent Application 55-24646. FIG. 7 is a sectional view showing the thus manufactured conventional high temperature operating element. First, a raw material for forming a ceramic substrate 10 is prepared and a heat generating layer 11 having a predetermined configuration is formed on a sheet by a printing technique such as extrusion through a roll or a casting method. Then, an insulator 12 is formed on the substrate 10 with the heat generating layer 11 formed thereon and then a cathode lead layer 13, a base metal layer 14 and a cathode material layer 15 are formed on this insulator 12 by the same printing method, so that the high temperature operating element is formed. The heat generating layer 11 is formed on the substrate 10 by screen printing a paste in which a baking assistant is applied to a heater material. The operating element is formed on the substrate 10 by screen printing a paste in which the baking assistant is applied to the desired material. After the screen printing, they are baked at a high temperature (1000.degree. C.-2000.degree. C.) and then the high temperature operating element is formed.
In this method, a high temperature treatment is performed during manufacture. Therefore, if the heater is used below this processing temperature, the change of resistance with time is small, so that it is stable at a high temperature for a long time as a heater. However, the pattern precision obtained by screen printing is unsatisfactory and it is difficult to control (reduce) the thickness of the heat generating layer 11. Therefore, the power consumption is large and the resistance varies widely amongst a plurality of heaters. Therefore, as a method for forming a pattern with high reliability, a PVD method (Physical Vapor Deposition) and a CVD method (Chemical Vapor Deposition) have been developed.
FIGS. 8(a) to 8(d) illustrate method for manufacturing the conventional high temperature operating element by a thin film forming method. First, a resistive (heat generating) film 20 and a high temperature element film 40 are uniformly formed on opposite surfaces of a planar ceramic substrate 10, respectively. Then, a predetermined heater pattern and an element pattern are formed by etching and a lead wire 50 is connected to the heater side, whereby the high temperature operating element is produced.
FIG. 10 shows a structure of an electron emitting apparatus produced by the thin film forming method as a example of the conventional high temperature operating element. First, a resistive (heat generating) film 20 for a heater and a film for a base metal 18 (reduction member) are uniformly formed on one surface and the other surface of a planar ceramic substrate 10, respectively. Then, a desired heater pattern and a pattern for a cathode are formed by etching and an electron emitting member 19 is applied to the base metal film. A lead wire 50 is connected to the heater side, whereby an electron emitting apparatus is produced.
A description is given of a method for manufacturing a conventional planar thin heater used in such a high temperature operating element. FIGS. 11(a) to 11(d) are process diagrams showing a method for manufacturing the planar thin heater by the conventional thin film forming method. For example, a resistive (heat generating) film 30 for heater is uniformly formed on a planar ceramic substrate 10 of Al.sub.2 O.sub.3 (FIG. 11 (b)), then a desired heater pattern is formed by etching (FIG. 11(c)) and then, a lead wire 50 is connected thereto (FIG. 11(d)). As a result, the planar thin heater is provided.
In the conventional high temperature operating element produced by the above method, the resistance changes while it is used as a planar thin heater with a voltage applied to the lead wire 50. This is because the resistive (heat generating) film 20 is thin. FIG. 9 shows a change of a resistance value of the heater with time. In FIG. 9, the ordinate designates a resistance value and the abscissa designates time. As shown in the FIG. 9, resistance falls at an early stage because the thin film is recrystallized and crystalline grains in the film grow in size. For example, when the resistive (heat generating) film 20 is W (tungsten) and it is used at 1000.degree. C., it is recrystallized because 1000.degree. C. is the recrystallization temperature of W. In addition, resistance increases with time because impurities enter the film from the ambient or the film is oxidized. Therefore, it is not stable as a heater and reliability over a long period of time is not guaranteed.
Since an oxide substrate such as Al.sub.2 O.sub.3 is readily available in a monocrystalline state and can be ground to a mirror finish, the patterning precision thereon is better than that of a sintered substrate such as SiC and AlN. However, in a heater using an oxide substrate such as Al.sub.2 O.sub.3 as shown in FIG. 11, a part of the substrate below the resistance wiring end is selectively damaged by thermochemical or electrochemical action caused by oxygen during its use. As shown in the photograph 3 showing a sectional view of an end of the conventional planar thin heater of the high temperature operating element after its use, this damage causes reduced heater life.
In addition, in the high temperature operating element such as an electron emitting apparatus provided by the thus described method, the film peels off the substrate 10 when a voltage is applied to the lead wire to heat the heater and the cathode is heated through the ceramic substrate 10 to emit electrons. More specifically, the resistive film 20 peels off the ceramic substrate 10 or the ceramic base metal film 18 peels off in the structure shown in FIG. 10. The reason for this is that an adhering force between the film and the substrate is originally weak, a change in a balance of an internal stress occurs due to the heating and cooling during its use and thermal expansion coefficients of the film and the substrate are different. Therefore, heat capacity changes due to the peeling off, the resistance value as a heater fluctuates, a wire breaks in the heater and the amount of electron emission from the cathode changes with the change of the heat capacity. Furthermore, the base metal (reduction member) film 18 does not well adhere to the cathode material and an electron emitting characteristics deteriorate, therefore the heater and the cathode are unstable and long-term reliability is reduced. Therefore, performance is not satisfactory.
As described above, the conventional high temperature operating element is formed alternatively by providing a porous film with low film density on both surfaces of a substrate by a thick film circuit forming technique or by providing a film with high film density and with a less adherence by a thin film forming method. However, these techniques do not produce satisfactory heater performance.